1. Field of the Invention
The present invention relates to the production of thin films and, in particular, to methods and apparatus for producing a thin film on a substrate by subjecting the substrate to repeated pulses of gas or vapor-phase reactants.
2. Description of the Related Art
Atomic Layer Deposition (abbreviated “ALD”), which was formerly called Atomic Layer Epitaxy or “ALE,” is a method in which thin films are grown on the surface of a substrate by subjecting it to alternate surface reactions of vaporized reactants. As known in the art, in conventional MBE and CVD processes, the growth rate of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate and the substrate temperature. By contrast, the thin film growth rate in the ALD process is controlled by the substrate surface properties, rather than by the concentrations or other qualities of the starting material inflows. In the ALD process, the only prerequisite is that the starting material is provided in a sufficient concentration for film growth on the substrate.
The ALD method is described, e.g., in FI Patents Nos. 52,359 and 57,975 as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Apparatuses for implementing the ALD method are also disclosed in U.S. Pat. Nos. 5,711,811, 5,855,680 and 6,015,590.
In the ALD method, atoms or molecules sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon by a single pulse in a self-limited manner. A separate pulse reacts with the monolayer, and the cycle repeats. Each cycle can include tow or more pulses of different reactants, each separated in time and space by removal steps (e.g., purging). Thus, typically, an ALD method comprises the steps of feeding vapor-phase reactants into a reaction chamber in the form of vapor-phase pulses repeatedly and alternately, causing the vapor-phase reactants to react with the surface of the substrate to form a thin film compound on the substrate.
Equipment for the ALD method and processes utilizing the ALD method are commercially available under the trade name of ALCVD™ by ASM Microchemistry Oy, Espoo, Finland.
ALD processing is usually carried out as a batch process. There are some apparatuses suggested for simultaneous processing of a plurality of substrates. Thus, one approach is presented in U.S. Pat. No. 5,879,459. The system of the reference consists of single wafer Low Profile Compact atomic layer Deposition Reactor (LP-CAR) compartments that are arranged in a vertical stack. Every LP-CAR contains a gate valve for the wafers to be transferred in and out from the process chambers with the cluster tool provided. The LP-CAR's must also provide equal process conditions for each wafer, and a significant number of heaters, gate valves, gas supplies and vacuum lines are required for batch processing.
Another set-up is presented in U.S. Pat. No. 6,042,652. There is a plurality of stages for holding substrates inside semi-open substrate compartments. The substrate trays are arranged so that reactants and inert gas flow over the parallel substrates. The reactant supply is balanced between substrate compartments by flow distributors that restrict the gas flow in a controllable way.
One reactor set-up that changes the flow direction of gases is presented in published PCT patent application WO 00/79576. The gas feeds and the exhaust are arranged on top of a single substrate reactor.
There are a number of disadvantages and problems related to the cited known art. A fundamental problem of conventional batch systems is their dependence on radiative heat transfer due to the poor conduction through the gas at lower pressures typically employed in ALD. At low pressures, the heat transfer from the heated surfaces to the substrates by conduction through the low-pressure gas and by the convection of the low-pressure gas is poor. The amount of heat energy transferred mainly by infrared radiation is based on the temperature difference between the emitting surface and the absorbing surface.
Furthermore, the reactor presented in U.S. Pat. No. 5,879,459 is expensive for batch processing of substrates because of the complex structure and the large number of costly parts. A problem particularly related to the apparatus of U.S. Pat. No. 6,042,652 is that stringent requirements must be placed on the tightness of the flow distributors to ensure that the distribution of gases is even for each substrate compartment.
In addition, the reactor presented in published PCT patent application WO 00/79576 can be used only for processing single substrates and the design with a plasma electrode is not meant for batch processing.
Therefore, based on the art cited above there is a need for an improved method and apparatus for ALD batch processing, which improves heat transfer, which reduces heat-up times and which increases the capacity of the equipment. There is also a need of simplified batch processing equipment.